Re: [avr-gcc-list] efficiency of assigning bits

[Royce & Sharal Pereira]

On Fri, 18 Mar 2005 22:29:30 +0530, E. Weddington <address@hidden> wrote:

> > > PORTD = (PORTD & (unsigned char)~(_BV(0))) | (unsigned   
> > > char)_BV(1)|_BV(2)|_BV(3));
> >
> >
> > Whew! What a long winded way to do things! This takes less time/energy  
> > to  enter:
> >
> > PORTD &= ~(1 | (1<<1) | (1<<2));
> 1. What you wrote above is different than what I wrote above. Follow the  
> parentheses carefully to understand why.

Exactly my point-readability! I correct my statement to:

PORTD = (PORTD & ~(1<<0)) | (1<<1) | (1<<2);
I feel this is still more readable.

> 2. In your opinion. Others may differ. In fact it is not always clear  
> which bits are getting set or why. If you use symbolic names in place of  
> the bit numbers, then it stands a better chance of being mnemonic. I  
> prefer not having "magic numbers" in the source code.
>
> For example, can you tell me offhand what this does on a mega128 without  
> looking at the datasheet?
>
> TIMSK |= (1<<5);
>
> No?
>
> However if you write it like this:
>
> TIMSK |= _BV(TICIE1);
>
> Then you can at least make out that this is setting the Timer 1 Input

Yes, in this case your point is valid. Bit names were not used however in  
the PORTD example.

> #define STATUS_LED    3
> //......
> PORTA |= _BV(STATUS_LED);
>
> What if the above code was for a prototype board, which then needed to  
> be re-layed out? What if the status LED needed to move to a different

I'm still comfortable with :
#define STATUS_LED    3
PORTA |= 1<< STATUS_LED;
I prefer standard C statements as much as possible. But like you said,  
it's my opinion. Thats also the reason I don't mourn the demise of the  
sbi() & cbi() macros ;)
> >
> 3. IIRC, a constant defaults to an *integer*, which on the AVR is 16  
> bits. It does not default to an unsigned char. So you still have the  
> same problem and you will still have to typecast the constants.

I dont know...check this code result (from the .lss file):
      PORTD= (PORTD & ~1) |(1<<2) |(1<<3);
 924:   82 b3           in      r24, 0x12       ; 18
 926:   8e 7f           andi    r24, 0xFE       ; 254
 928:   8c 60           ori     r24, 0x0C       ; 12
 92a:   82 bb           out     0x12, r24       ; 18

--Royce.
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